As the demand for energy has increased and the supplies of fossil fuel dwindled there has been a renewed look by electrical utility companies at alternative methods for producing electrical power. One method of electrical production involves the harnessing of the wind by a wind turbine to drive an electrical generator.
Wind turbines typically involve using a series of blades fixed to the top of a tower to rotate about a horizontal axis. The blades have an aerodynamic shape such that when a wind blows across the surface of the blade, a lift force is generated causing the series of blades to rotate a shaft about an axis. The shaft is connected, typically via a gearing arrangement, to an electrical generator located in a structure called a nacelle positioned behind the blades. The gear box converts the rotation of the blades into a speed usable by the generator to produce electricity at a frequency that is proper for the electrical grid.
Alternatively, a wind turbine may use a direct drive permanent magnet generator. This configuration has the advantage of eliminating an expensive and low reliability component, namely the gear box. A typical high-speed generator, such as that used with a gear box, will have a rotor with permanent magnets and a solid core. Due to the high speed of the rotor it is only feasible to have a small number of poles at a relatively small diameter. For low-speed generators, such as that used in a direct drive wind turbine, a larger diameter and/or more poles are needed to generate power.
These large diameter rotors are often hollow in order to conserve material and reduce weight. Permanent magnet rotors can additionally be split into two types: those with magnets mounted on the surface of a magnetic steel rim; and those with magnets interspersed between magnetic steel rotor poles. The disadvantage of the steel rim is that the magnets are positioned very close to the stator. Due to the heat generated by the stator, heat damage may result to the magnet and cause it to lose its magnetic properties. The rotor type with steel poles provides better protection of the magnets from the heat, but the rim must be made from a non-magnetic material. While an aluminum materials may work well for a small diameter rotor, the use of stainless steel is usually required in the megawatt range to avoid thermal expansion issues. Additionally, stainless steel is often prohibitively expensive when compared to magnetic steel such as plain-carbon steel.
Another common feature of permanent magnet rotors is to use a wedge to hold the magnets in place. A plate is then fixed at each end to retain the magnets and wedges. Only one plate must be removed in order to install or remove the magnets and wedges. When the rotor is assembled, typically dummy magnets, made from a nonmagnetic metal, are utilized during assembly in place of the magnets. The dummy magnets are used in order to evenly locate the rotor poles before tightening the bolts. The dummy magnets are sized slightly larger than the permanent magnets so that the magnets will slide easily into place.
Accordingly, it is considered desirable to provide a rotor which utilizes non-stainless steel structural members to lower cost while minimizing the effects of thermal expansion during operation. Additionally, it is also desirable to provide an integrated rotor design that eliminates the need for dummy magnets during assembly.